ok ladies and gents.. I have a hard time controlling my speed on my old machines with no speed control.. I find that when I fmq the speed just gets away from me.. My dh bought one of these dimmers locally and plugged it into my singer and what a blessing it is ! I can now safely fmq without my foot running away with the foot controlled speed.
The picture is taken next to a krups cup for coffee makers to show you the size of it...(don't ask,,dh thought it was a good comparison)..

It regulates the voltage going thru the pedal to the motor. Just like a dimmer switch for your lights in the home. Your machine plugs into the dimmer switch,,the dimmer switch plugs into the wall. therefore it regulates the voltage going thru the cord to your pedal only allowing enough speed set on the dimmer switch.

I actually plugged it into an extension cord so that it could be on the table next to me for adjustment.. you have to set your speed and then when you use it for fmq or piecing or whatever.. you need to push the floor pedal all the way down to get your adjusted speed.
the speed was adjusted with no thread involved. so it didn't bind or get caught up while "playing".